The present invention relates to a linear charged particle and in particular ion accelerators, comprising drift tubes. This accelerator which can in particular accelerate two types of ions having different masses, can be used in the production of radioactive elements for medical use, in the construction of ion probes, in isotopic dating and in the construction of high energy ion implanters.
FIG. 1 shows in exemplified manner the circuit diagram of a standing wave linear accelerator having Wideroe-type drift tubes. This accelerator comprises a generally cylindrical cavity 1, in which are arranged along axis 2 thereof, tubes 4 and 6, which are currently called drift tubes, which define between them gaps I. These tubes 4, 6 are alternately connected to the two terminals of a high frequency generator 8. The ions, injected by a source 10, are accelerated in gaps I by the high frequency electrical field prevailing therein.
It is known that accelerating structures or linear accelerators with drift tubes cannot be used with advantage for accelerating ions whose charge q/mass A ratio varies only slightly from the optimum value for which they were designed.
Thus, in such devices having a certain number of drift tubes, the law of particle velocity is imposed. The electrical field necessary for accelerating the ions is consequently inversely proportional to the q/A ratio. A device designed for accelerating particles of ratio (q/A).sub.o, with the maximum electrical field will be unable to accelerate particles such that q/A is below (q/A).sub.o and particles for which q/A exceeds (q/A).sub.o cannot be accelerated to an energy per nucleon which significantly exceeds that obtained with the particles of ratio (q/A).sub.o.
The different methods proposed for obviating this problem, such as regulating the frequency of the electrical field, modifying the position of the drift tubes, etc. have the disadvantage of considerably complicating the technological construction of the accelerating structures and of consequently making them less reliable and more expensive.
In the aforementioned standing wave accelerating structures, it is also known that the spatial period L of the structure (length of a tube, plus length of a gap) is proportional to the wavelength in vacuum .lambda., associated with the electrical field, and with the ratio .beta. of the velocity of the ions to that of light. More specifically, in Wideroe accelerators, such as are diagrammatically shown in FIG. 1, the spatial length L is governed by the equation L=.beta..lambda./2. In the same way, the external diameter of the drift tubes is proportional to the wavelength .lambda. and the ratio .beta..
To ensure that the mean value of the electrical field is not too low, compared with its peak value, it is virtually necessary to choose for the length of the acceleration gaps I, a value close to that of the drift tubes, i.e. close to .beta..lambda./4 in the case of the Wideroe type.
Moreover, to ensure that the electrical field is sufficiently homogeneous in the acceleration gaps I, the external diameter of the tubes must not be too small compared with the length of the acceleration gaps. Generally this diameter has a value close to that of the length of a gap I, i.e. is more than half .beta..lambda./2 and close to .beta..lambda./4.
Thus, for high values of (above about 0.15), it is necessary to have drift tubes with an unnecessarily large diameter compared with that necessary for the passage of the beam.
The capacitive load of the drift tubes consequently becomes very high. The currents circulating in the walls of these tubes are then intense and lead to a prohibitive energy dissipation. Thus, the effective linear shunt impedance Z of these structures, defined by the equation Z=E.sup.2 /P.sub.1, E being the mean value of the electrical field and P.sub.1 the power dissipated by unit of length, becomes much too low.